A server cluster is generally a group of servers (nodes) arranged such that if any server fails, the other servers of the cluster can transparently take over the work of the failed server, that is, restart its applications and thereby continue serving clients without significant interruption. This operation is generally referred to as failover, or failover clustering.
In a cluster, each storage device (hard disk drive) is owned by one node. The owner node, if properly operating, defends ownership of the storage device against other nodes' challenges for ownership, so that another node can take over ownership if the owner fails.
To implement such a challenge-defense model, a persistent reservations ownership algorithm us used, as described in U.S. Pat. No. 7,277,952. However, this persistent reservations ownership algorithm implements a share-nothing model in which the node owning the disk has exclusive ownership and full access to the disk, while other nodes have read-only access.
One aspect of cluster shared storage volumes technology (described in the related patent application Ser. No. 12/147,956) is that other healthy cluster nodes need to be able to simultaneously write to shared cluster disks. As a result, a way to grant access of the storage device to healthy nodes, while excluding any nodes that are not properly participating in the cluster, is needed.